It is known to convert the infrared radiation of a laser to the visible spectral range with the help of optical, non-linear crystals preferably arranged inside the resonator. Such lasers can emit the desired pulses in a Q-switched mode.
However, Q-switched solid-state lasers have an upper limit of the pulse repetition frequency, which is determined, for instance, by the lifetime of the fluorescence of the upper laser level, the stimulated effective emission cross-section of the laser ion, the length of the resonator, the degree of coupling-out and the pumping power density. Above this limit frequency, strong fluctuations in pulse energy occur initially between two subsequent pulses (“ping pong effect”), with every other pulse respectively having the same pulse energy, but, in an alternating manner, each pulse of a higher pulse energy is followed by a pulse of a low pulse energy. In the case of still higher pulse repetition frequencies, every other pulse drops out, or there may even be several bifurcations with respect to the pulse energy. Thus, on the whole, operation above this limit frequency no longer makes sense from a technical point of view. U.S. Pat. No. 6,654,391 describes a method for a Q-switched laser with frequency doubling inside the resonator, wherein pulse stabilization is achieved in that the pulse tail of the frequency-doubled laser radiation is respectively cut off on the descending slope. What is essential here is that part of the stored energy remains in the laser, thus achieving an improvement of the pulse-by-pulse stability at high pulse repetition frequencies and an increase in frequency doubling. However, substantial pulse shortening is not possible and, therefore, this method is suitable only for Nd:YAG or Nd:YVO4 or comparable systems having a short-lived upper laser level and large effective amplification cross-sections, which lead to shorter pulses in a Q-switched laser. In order for this method to achieve high power averages for high beam quality, Nd-doped lasers are unsuitable because the high quantum defect causes considerable heating of the laser crystal and thus opto-thermal interferences to occur, limiting the power output in the case of high beam quality.
These limitations do not exist in the case of Yb:YAG lasers in the disk laser arrangement as described, for example, in EP 0 632 551. However, Yb:YAG is characterized by a very long life of the upper laser level of approx. 1 ms and by a small effective amplification cross-section. In Q-switched operation, the pulses become unstable at pulse frequencies of more than 25 kHz and the pulse lengths may be up to several μs.
U.S. Pat. No. 4,841,528 discloses a laser assembly wherein the laser is operated in the cavity dumping niode, with the coupled-out laser radiation being frequency-doubled by means of a non-linear crystal which is arranged outside the resonator. The assembly is provided such that the part of the coupled-out laser radiation which is not frequency-doubled is coupled into the resonator again. An arrangement wherein the frequency-doubled crystal is arranged within the resonator is described as disadvantageous in this reference.